1. Field of the Invention
The present invention relates to a laser module and more particular to a laser module used as a light source for sending/receiving digital signals.
2. Description of the Related Art
A light transmission module is capable of transforming an electrical signal to an optical signal of a desired wavelength through a semiconductor laser. A light reception module is capable of transforming the optical signal to the electrical signal.
In order to transmit digital signals of Non Return to Zero (NRZ), Return to Zero (RZ), etc., a laser diode—such as a 2.5 Gbps Distributed Feedback-Laser Diode (DFB-LD) module or a 10 Gbps DFB-LD module—is used as a light source. In particular, a laser module of a 1.55 um beam wavelength is preferably used for transmitting signals in a long distance away due to the low fiber loss after transmission.
Basic elements constituting the laser module include a laser diode, proper transmission lines for RF-signal, an optical system for collimating the optical signal into fiber. A Monitor Photo Diode (MPD) is also included for monitoring the light output of the laser diode in order to maintain its stability.
The laser diode generates an optical signal in response to an input signal via a lead pin connected to an external integrated circuit, while the MPD detects the average light intensity from the laser diode. The optical system contains lenses to play the role of collimating light. The collimated light is transmitted through the optical fiber, which is attached to the optical system. During operation, an undesirable overshoot of light intensity occurs when the state of laser diode transits from off-state (0-level) to on-state (1-level). This undesirable overshoot is caused by the relaxation oscillation phenomena by the laser diode which can be represented by a relaxation oscillation peak of the frequency response of a laser diode, as shown in FIG. 1. The relaxation oscillation frequency, fr, can be defined by the frequency of maximum response. As the relaxation oscillation frequency, fr, increases when the driving current of the laser diode increases, the fr at on-state (1-level) is larger than the fr at off-state (0-level). Further, as the frequency chirping increases as the intensity overshoot grows, a laser module of a 1.55 um beam wavelength with large overshoot has the problem of shorting the optical transmission distance.